docs(test): clarify reason for skipped CommonMark roundtrip tests

[satyaij6]

Description

This PR clarifies the reason why the CommonMark roundtrip tests are currently skipped.

The previous comment indicated that the tests “need more investigation” without explaining the nature of the issue. Based on initial investigation, the failures are due to known whitespace and normalization differences in Markdown → AST → Markdown transformations, rather than semantic errors.

This improves test readability and helps future contributors understand the limitation without re-investigating it.

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Changes

• Replaced vague TODO comment with a clear explanation of the normalization issue.
• Updated the skipped test name to make the reason visible in test output.

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Notes

• Documentation-only update.
• No functional or behavioral changes.
• All tests pass with expected skips.

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Related Issue

• Relates to Some CommonMark spec examples do not roundtrip #2

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Checklist

• Changes are limited in scope and non-breaking
• Commit messages follow project conventions
• Tests run successfully